. . . . .

In the two above snippets I am wondering how "b)" was arrived at. In the question both parts of the equation show a plus and at "b)" one of the signs has changed to a minus symbol. I'm really not sure what is meant by generalizing to an arbitrary function as I am being asked to do.

Thanks in advance...

original images:

https://skydrive.live.com/redir?resid=D ... gpF0ko&v=3

https://skydrive.live.com/redir?resid=D ... yVVJws&v=3

I'd guess it's just a typo. The formula doesn't make sense without the "-", but it does make sense and it does work the other way.